1. Field of the Invention
The present invention relates generally to a method for preparing particles of pyrithione salts, and more specifically to a method of preparing submicron-sized particles of pyrithione salts under pressurized, turbulent conditions. The present invention also relates to particles made by the method, as well as products made with the particles produced by the method.
2. Description of the Related Art
Polyvalent metal salts of pyrithione (also known as 1-hydroxy-2-pyridinethione; 2-pyridinethiol-1-oxide; 2-pyridinethione; 2-mercaptopyridine-N-oxide; pyridinethione; and pyridinethione-N-oxide) are known to be effective biocidal agents, and are widely used as fungicides and bactericides in paints and personal care products such as anti-dandruff shampoos. The polyvalent metal salts of pyrithione are only sparingly soluble in water and include magnesium pyrithione, barium pyrithione, strontium pyrithione, copper pyrithione, zinc pyrithione, cadmium pyrithione, and zirconium pyrithione. The most widely used divalent pyrithione salts are zinc pyrithione and copper pyrithione.
Zinc and copper pyrithione are useful as antimicrobial agents active against gram-positive and negative bacteria, fungi, and yeasts. Zinc pyrithione is used as an antidandruff component in shampoos, while technical suspensions of zinc pyrithione and/or copper pyrithione are used as preservatives and antifouling agents in paints and polymers. Synthesis of polyvalent pyrithione salts are described in U.S. Pat. No. 2,809,971 to Berstein et al. Other patents disclosing similar compounds and processes for making them include U.S. Pat. Nos. 2,786,847; 3,589,999; 3,590,035; 3,773,770.
Known methods for producing insoluble polyvalent salts of pyrithione result in large solid particles having an average size greater than 2 micrometers (.mu.m). However, smaller particles of pyrithione salts (i.e., less than 1 micrometer or submicron) are often desired because they more easily form suspensions and provide a larger surface area for enhanced biocidal activity. In addition, smaller particles, particularly in the low submicron range (e.g., below about 0.2 .mu.m), are transparent to light, and thus provide the opportunity to manufacture "clear" products, such as clear shampoos and soaps, that are popular in the marketplace today.
Smaller particles of pyrithione salts are usually generated by a separate mechanical manipulation step (e.g., grinding or crushing) on larger particles or crystals that are made by conventional processes. For example, European Patent Application No. 70046 describes preparation of zinc pyrithione using organic solvents. This process results in production of large crystals of zinc pyrithione that are easily isolated by filtration. A separate, optional grinding step is used to grind the large crystals and produce zinc pyrithione particles of smaller size.
As another example, U.S. Pat. No. 4,670,430 describes a process of making zinc pyrithione particles with a median size of about 0.2 .mu.m by grinding larger zinc pyrithione particles.
There is currently no single step method for producing particles of pyrithione salts in the submicron size range. The separate step of grinding larger particles to produce smaller particles, as described in the prior art, generally results in substantial loss of useful product, and is costly in terms of equipment, time, and energy required. Accordingly, what is needed in the art is a method to prepare pyrithione salt particles in the submicron size range without a separate mechanical crushing or grinding step. The present invention is believed to be an answer to that need.